The invention concerns a method for the deposition of wear resistant coatings onto parts under high vacuum with plasma assisted physical vapor deposition. The carrying out of this method requires a particular type of equipment which is also the subject of this invention.
Many methods for the physical vapor deposition were proposed over the last 30 years. Many of them have found widespread application since. (see E. Bergmann and E. Moll: plasma assisted PVD coating technologies published in Surface Coatings and Technologies volume 37 (1989), pages 483 ff.). All these methods can be described as a combination of 3 process steps: conditioning, deposition, deconditioning. Conditioning comprises in most cases several conditioning steps: cleaning, putting under high vacuum, heating and plasma etching. Deconditioning comprises in most cases the deconditioning steps: cooling, removing from high vacuum and conservation. This sequence of steps is used in almost all methods for the plasma assisted high vacuum physical vapor coating of parts with wear resistant coatings. Exceptions are of course the coating of temperature sensitive parts, where one skips heating. Parts are considered as temperature sensitive, if they are not to be heated without damage to more than 650.degree. K. The state of the art of conditioning for methods for the plasma assisted physical vapor coating of parts with wear resistant coatings has been described in the patent applications DE 3936550 and DE 104998. These two patent applications recommend putting under high vacuum and radiation heating. In the case of radiation heating heat flows as a beam of infrared photons from a heater to the parts to be heated. A heater is a surface, whose temperature is higher than the temperature of the parts to be heated. The set point temperature is the temperature the parts should reach in the conditioning step. The different variants of plasma etching are not discussed being not a subject of this invention. All plasma etching methods used today are executed under high vacuum although one could conceive rough vacuum methods.
Any physical vapor deposition can be considered as sequence of 3 processes each of them being stationary in time: Evaporation of components of the material that will form the coating in a suitable installation, called source. Transport of these components that will form the coating and, if appropriate, gaseous components to the parts. Conversion of these components on the surface of the parts to coatings with the required properties. Numerous forms of vapor sources are known and used today. (see E. Bergmann and E. Moll op. cit.). In the case of the physical vapor deposition of wear resistant coatings they are based either on sputtering or on arc evaporation. The evaporated components for forming the coating are transported to the parts by a free molecular flow or by means of an electrostatically and/or electromagnetically managed molecular flow. Thereby a mass flow of components of the material constituting the coating is formed. The following transport configurations have been realized so far, each being specific to a certain equipment and vapor source configuration:
A flat source facing a flat part: Not suitable for wear protection coatings, where most parts are complex shaped. Mainly used in load-lock systems.
Flat sources on a cylinder and a radial mass flow to the parts in the center of the cylinder.
Moving point sources or rod sources in the center and radial mass flow towards the components on the cylinder surface.
Point source in the center or on the bottom and radial flow to the substrates mounted onto the segment of a sphere.
Flat sources are evaporation installations, where the components of the material that will form the coating are emitted from an extended surface and where this surface is flat. Point sources are evaporation installations where the components of the material, that will form the coating are emitted from a surface whose extension, typically in the range of 0.001-0.003 m, is very small compared to the vessel surface. Rod sources are evaporation installations, where the components of the material, that will form the coating are emitted from a rod.
State of the art methods use radiation also for cooling of the parts.
State of the art methods for the plasma assisted high vacuum physical vapor coating of parts with wear resistant coatings use transport configurations with either parallel heat and mass flows or heat and mass flows, that are coradial. Coradial means being both in the radial direction of the same cylinder.
The reasons for the restriction to these transport configurations are considered evident. Under high vacuum heat flow in the form of a photon beam as well as mass flow of the components, that will form the coating, in the form of a molecular flow are directed flows. There sequential combination requires therefore a parallel or coradial direction of heat flow and mass flow, to assure equal uniformity of exposure of the parts to both flows. Since heat transport in high vacuum is limited to photons, the state of the art is limited to radiation heating.
The use of radiation heating brings many disadvantages in the practical application of these methods. Heat transport from the radiating surfaces to the core of the parts is poor, because it depends strongly on the surface finish of the parts and non-uniform temperatures can not be avoided: shading leaves some parts too cold, intensive irradiation overheats some parts. These problems arise from the fact, that the heat flow associated with radiation depends very strongly on the temperature difference between heater and part to be heated. The heat flow is proportional to the 4th power of this temperature difference. FIG. 1 shows the temperature evolution of 3 parts with different weight in different areas of an equipment with conditioning according to the state of the art. Curve (a) was measured with a twist drill made from H2 high speed steel, diameter 6 mm, fixed at half-height of the part carrier on a spindle at its periphery, loaded in a quiver executing a further rotation around its axis. Curve (b) was measured with a milling cutter, diameter 150 mm, length 200 mm, also fixed at half height of the part carrier on a spindle but free standing on a holder plate. Curve (c) was measured with a forming punch, diameter 300 mm, sitting in the center of the part carrier. The temperature of the heater was identical in all three experiments, namely 1270.degree. K. The target temperature for all three parts was 770.degree. K. The milling cutter reached this temperature after 2.5 hours. By that time the temperature of the twist drill had long exceeded his tempering temperature--810.degree. K., the drill had softened and was scrap. The punch never reached his target temperature. This lower than specified temperature of the punch during the subsequent coating step affected the adhesion of the plasma assisted coating adversely. In this experiment a heater temperature largely exceeding the target temperature of the parts had been chosen. In this way the light parts respectively the parts closer to the heater did exceed the target temperature and had approached the temperature of the heater, while heavy parts respectively parts at a larger distance from the heater had remained significantly below the target temperature. If one sets a small difference between heater temperature and target temperature, the heat flow becomes very small and the heating time excessively long.
These problems prevent currently the profitable coating of heavy parts with plasma assisted physical vapor deposition by job coaters. They also require from the operators of such equipment great skill in arranging mixed batches. The effect of the parallel or coradial arrangement of heat and mass flows leads to a heating of the part of similar non uniformity than the coating.